Multiple solvent secondary recovery process



United States Patent MULTIPLE SOLVENT SECONDARY RECOVERY PROCESS Edward F. Johnson and George G. Binder, Jr., Tulsa, Okla, assignors to Esso Research and Enganeering Company, a corporation of Delaware No Drawing. Application July 22, 1953, Serial No. 369,706

5 Claims. (Cl. 1669) This invention concerns the recovery or production of petroleum oils from underground reservoirs. The invention is a secondary recovery technique for securing essentially complete recovery of petroleum oil, utilizing in a particular manner a combination of two or more volatile solvents. In a specific adaptation of the invention a secondary recovery drive is employed depending on the sequential injection of n-butane, followed by propane, followed by a nitrogen gas drive.

A number of methods of recovering petroleum oil from underground reservoirs are now known. In'many cases reservoir pressures are sufliciently high so that oil can be produced during a primary production period by natural depletion of the reservoir pressure. vIn this case, the reservoir pressure serves to force oil from the formation so as to flow into a producing well. As the reservoir pressure becomes depleted it may become necessary to pump the oil through the producing well to the surface. In other cases the initial reservoir pressure may have been so low as to require use of pumping energy from the beginning of oil production. Re-pressuring of petroleum reservoirs by injection of re-pressuring agents such as natural gas is often employed to maintain higher reservoir pressures during production. Re-pressuring serves to lengthen the productive period by decreasing the rate of pressure depletion.

In oil production by these general techniques, it is now appreciated that in general not more than 50% of the oil originally in the reservoir can be recovered. Consequently, there have been many proposals of other techniques which could be expected to increase the ultimate oil recovery. Secondary recovery operations have been suggested for this purpose. Generally, secondary recovery procedures may be defined as utilizing the injection of a drive agent which is forced as a fluid through the reservoir to produce oil from other production wells. A number of secondary recovery drive agents have been suggested. At the present time, in actual use, water is predominantly employed in secondary recovery operations. However, while injection of water as a drive agent serves to increase oil recovery, beyond that possible by primary production methods, a great deal of oil still remains in place after it becomes uneconomical to continue ater flooding. gas drive utilizing a noncondensible gas such as air or Again, however, use of a gas driye' in secondovery procedures does not permit complete recovery of oil from a reservoir. As a rule of thumb, it has been stated that while primary recovery techniques permit recovery of about 50% of the oil, water or gas drive secondary recovery procedures can increase ultimate recovery to about two-thirds of the oil originally in place. The need therefore exists for an economically practical way to secure recovery of the oil in a reservoir which cannot be recovered by techniques which are now known.

A potential solution to this problem which has received consideration is the possibility of using a solvent for oil It has also been proposed to employ a erase "ice as the secondary recovery drive agent. A solvent such as propane or butane, for example, would serve in part as an extraction agent so as to theoretically improve the ultimate oil recovery. The potentialities of this technique however, are seriously limited by practical or economic considerations. It has been found that the solvent requirements to obtain complete oil recovery are prohibitively excessive. It becomes impractical to inject sufiicient solvent to attain complete oil recovery due to the expense of the large volumes of solvent required.

The maximum amount of solvent which can practically be employed in secondary recovery operations is about 0.3 pore volumes of solvent. In other words, an amount of solvent can be contemplated which is about one-third of the total pore volume of the particular oil reservoir. Use of quantities of solvent beyond this limitation become impractical for the reason given. One of the most promising secondary recovery methods of this nature, therefore, would employ the injection of up to about 0.3 pore volumes of solvent, followed by a drive agent such as water or gas. Such a procedure offers the greatest oil recovery for the amount of solvent employed. Nonetheless, however, it is now appreciated that complete oil recovery cannot be attained in this system.

It is not the purpose of this specification to exhaustively consider the theoretical aspects of secondary recovery processes. However, in order to secure a thorough understanding of the principles and scope of this invention, it is helpful to consider certain basic principles of secondary recovery operations in a more or less qualitative manner. As worked out above, the best secondary recovery procedure will in all likelihood employ a solvent as the primary driving agent. However, the use of a solvent must be supplemented with a second drive agent so as to minimize the quantities of solvent necessitated. Such a process could employ sequential injection of a solvent such as propane followed by injection of a drive agent such as air or nitrogen. The bank of propane solvent driven by the gas drive would serve to increase oil recovery beyond that obtainable by conventional water or gas drives. Consider now the limitations of this type of secondary recovery operation.

At an early phase of the secondary recovery operation, three distinct zones may be identified in the oil reservoir. In the bulk of the reservoir will be a first zone constituting the oil which it is desired to produce. A bank of propane solvent will constitute a second zone with a more or less distinct interface between the propane and the oil. Finally, a third zone will constitute the gas drive agent such as air or nitrogen with a more or less distinct interface between this zone and the propane solvent. As injection of the gas drive continues, these zones will be moved progressively through the reservoir towards a produeing well or wells. At the same time, changes will occur affecting the nature and composition of the three zones identified. Thus, at the propane gas interface, there will be continuous volatilization of propane. This will occur by virtue of the low partial pressure of propane in the injected gas. vaporization of propane will therefore occur permitting the gaseous propane to interfinger or penetrate the oil zone. As this occurs, propane will tend to condense in the oil. This condensation of propane into the oil zone is effective in increasing the saturation and flowability of the oil. It is this mechanism which permits the effective solvent extraction of oil from the reservoir. Nevertheless it is apparent that as this process continues the bank of propane will become depleted and in time the zone of propane will become completely exhausted.

When this occurs the zones of gas, solvent, and oil present in the reservoir may be identified as follows: moving back in the reservoir from a production Well to an injection well, adjacent the production well will be an oil zone into which very little propane has yet penetrated. Moving towards the injection well through the reservoir the concentration of propane in the oil will increase. Finally, there will be a more or less distinct evaporation front separating the oil rich in propane from the dead oil. As stated, there will be no zone remaining at this period of production which is occupied solely by propane. When this condition has come about it follows from what has been said that complete oil recovery can no longer be achieved. Since a gas drive is incapable of permitting complete oil recovery, this oil which has been depleted of propane cannot be recovered in this secondary recovery procedure.

From what has been said, it is apparent that in a solvent extraction type of secondary recovery operation, it becomes essential to prevent exhaustion of a distinct solvent zone. Because of the limitation on the amount of solvent which can practically be employed, it has theretofore been impossible to prevent exhaustion of the solvent bank before the gas drive overtakes residual oil. For this reason complete oil recovery has heretofore been unobtainable. It is one purpose of this invention to provide a solvent extraction secondary recovery process which can prevent a gas drive from overtaking residual oil before exhaustion of the solvent bank. This is achieved in a manner which greatly minimizes the quantity of solvent which must be employed in the forefront of the gas drive in order to achieve this object.

As worked out above, in a gas driven solvent extraction secondary recovery process, the drive mechanism occurs by virtue of the constant volatilization and condensation of the solvent employed.

This invention utilizes what may be considered as a guard solvent bank between the oil to be displaced and a solvent drive. For this purpose, a dual solvent system is employed wherein the two solvents are of different volatility. This creates the possibility and necessity for one solvent to vaporize and condense in the other solvent, and to then again vaporize and condense in the oil being driven, before a gas drive front can reach any oil. The proven superiority of the secondary recovery process of this invention is believed to be due at least in part to this general mechanism.

In accordance with this invention therefore, in a secondary recovery process, a first solvent such as butane is injected to a reservoir. Thereafter, a second bank of solvent such as propane is injected behind the butane. Finally, a conventional gas drive is employed to drive the two banks of solvent. In this system as the drive proceeds there will be vaporization of the butane with consequent condensation of butane in oil. At the same time there will be vaporization of propane followed by vaporization of propane in butane. Again, there will be vaporization of propane from the propane-butane mixed phase, followed by condensation of this propane in the oil. While this process has been identified by reference to the specific solvents, propane and butane, other solvents may be chosen from a limited range of possibilities. Generally the solvents employed may be selected from the C3 to C5 hydrocarbons. While it would theoretically be possible to select certain chlorinated compounds and the like of suitable volatilities to be used in place of these hydrocarbons, such a possibility is practically undesirable due to expense. As stated, therefore, the solvents to be employed are two solvents differing in volatility selected from the group of C3 to C5 hydrocarbons.

It is apparent that the benefits of this invention may also be achieved when employing three or even four solvents by sequentially injecting each of these solvents. However, it is presently contemplated that there is little need to use three rather than two solvents to achieve the objectives of this invention.

When employing this invention, certain factors must be critically observed. It is necessary to operate at reservoir pressures above the normal vapor pressure of any of the solvents employed. This is essential in order to provide a liquid bank of the solvent to permit the general vaporization-condensation phenomena discussed formerly. As is well known, the pressure in a reservoir can be satisfactorily controlled by throttling of production wells and by control of the injection pressure. In many cases, however, the invention may be applied to reservoirs which have sufficient natural pressure. In any case, the pressure in the reservoir during conduct of this process must generally be above about 200 lbs. per square inch.

A second critical factor in the process of this invention is that the least volatile solvent should be injected first. Thus, if propane and butane are the two solvents to be employed, the butane would be injected first, followed by the propane. Again, if butane and pentane are the solvents chosen, then pentane would be injected first, followed by butane. The benefits of this invention cannot be achieved by injecting these solvents in the opposite order. As indicated, better drive efficiency is obtained because of the sequential vaporization and condensation of the last injected solvent. This depends in part on the fact that the more volatile solvent travels faster through the reservoir so that it can overtake the first injected solvent. However, if the order of injection of the solvents were to be reversed, this mechanism could not be achieved.

The gas to be employed in driving the multiple solvent system through the reservoir is broadly any gas which is non-condensible at the reservoir conditions. For practical reasons natural gas and nitrogen are preferred for use. Nitrogen, because of its higher viscosity, is a somewhat better drive gas than is natural gas.

The nature and advantages of this invention may be understood by reference to the following specific examples. These examples are based on laboratory flooding experiments utilizing sandstone cores arranged to permit controlled conduct of flooding experiments. One particular core employed was a sandstone having a per meability of 10 md. and was a core about 18 feet long. This core was mounted horizontally in a manner permitting a linear flow of fluid from one end of the core to the other. The core was cleaned and conditioned for use by the following steps: (1) Removal of all water and oil-soluble material; (2) Establishment of connate water saturation; (3) 1.3 cp. oil injected; (4) Primary production simulated by driving with gas. Thereafter, different solvent systems, as will be identified, were injected at one end of the core, and the oil production from the other end of the core was determined. In all cases an injection pressure of about 375 lbs. per square inch was used and the temperature maintained was about 75 F.

In a first experiment a volume of propane in liquid form was injected to the core. This propane had a volume equal to 20% of the original hydrocarbon volume (H. V.) of the core. The propane was driven through the core by means of a nitrogen drive. Using propane as the extraction solvent in this manner, it was found that the residual oil content of the core could be reduced to 11.8% H. V.

A similar experiment was conducted employing butane and it was determined that the extraction efii'ciency bf butane was virtually the same as propane.

Finally, a run was conducted embodying the principles of this invention. In this case 10% H. V. of butane was first injected, followed by 10% H. V. of propane. These solvents were passed through the reservoir 'by means of a nitrogen drive. It was found that the residual oil content of the core was reduced by this means to 5.1% H. V. It will be observed that while the same total volume of solvent was employed, the dual solvent system was appreciably more effective than either of the single solvents in removing oil from the core.

entries 'In order to obtain the benefits of this invention by use of a dual solvent bank, it has been found essential to employ more than a critical lower limit of solvent. in the event that less than about 15 percent hydrocarbon volume of solvent is employed, the use of two solvents totalling that quantity shows no advantage over the use of a single solvent of that quantity. The specific amount of solvent for which it makes no difference whether a single or a dual solvent bank is employed is dependent on the particular reservoir. However, in general, if 15 percent hydrocarbon volume or less of solvent is employed, there is no advantage in employing a dual solvent system. However, when more than 1 5 percent hydrocarbon volume of solvent is to be used, the dual solvent system of this invention provides a substantially higher degree of oil recovery.

This is shown by the following data:

A run in a linear Bendera sandstone model with 7 /2 percent normal butane followed 'by 7 /2 percent i-sobutane gave a residual oil saturation of 9.1 percent H. V. This compared with substantially the same residual oil content when '15 percent H. V. propane was employed. Another run in :a 5-spot model of Bartlesville sandstone using 15 percent propane gave a residual oil saturation of 26.5 percent H. V. This result can be compared with a similar run in the same model in which the solvent bank was made up of 7 /2 percent H. V. normal butane followed by 7 /2 percent H. V. propane. In this case, the residual oil saturation was 27 percent H. V. This residual oil content is considered identical to that in the previous run. When 30 percent H. V. propane was used in this 5-spot model of Bartlesville sandstone, however, a residual oil content of 22.9 percent was found. When using a dual solvent bank composed of 15 percent H. V. butane and 15 percent H. V. propane in the same model, an appreciably lower residual oil saturation of 17.0 percent was found.

In accordance with this invention therefore, in secondary recovery operations at least two solvents are to be injected into a reservoir for drive purposes. The first of these solvents will be the least volatile solvent, and will be injected in proportions of about 0.05 to 0.2% H. V. The second solvent injected will be the more volatile of the two solvents and will be injected in an amount approximately equal to that of the first solvent injected. Thereafter, any desired gas will be injected as the drive agent to drive these banks of solvent through the reservoir.

It is apparent that this invention may be employed with any desired configuration of injection and production wells according to principles appreciated in secondary recovery operations.

What is claimed is:

1. A secondary recovery process for recovering oil from an underground oil reservoir which is in communication with an injection well and a producing well which comprises injecting a first oil solvent down the injection well and into the oil reservoir, said firs .asolvent being a hydrocarbon containing trom 3 to carbon atoms, thereafter injecting a second oil solvent down the injection well and into the reservoir, said second solvent also being a hydrocarbon containing from3 -tb 5 carbon atoms; the combined amounts of the two solvents being greater than 15 percent by volume of the hydrocarbon pore volume of the reservoir and the second solvent being more volatile than the first solvent, maintaining the pressure of the reservoir at a value greater than the vapor pressures which the solvents normally possess under the conditions that prevail within the reservoir, therea; jecting a drive gas -down the injection well into the oil reservoir, said gas beiiig non-'condensible under the conditions that prevail Within the reservoir, and removing from the reservoir through the producing well .oil that is swept from the reservoir.

2. A process :as defined in claim 1 in which the volume of each solvent injected constitutes about 5 percent to 20 percent by volume of the hydrocarbon pore volume within the reservoir.

3. A process as defined in claim 1 in which the volume injected of said first solvent is about equal to the volume injected of said second solvent.

4. A process as defined in claim 1 in which the first solvent is normal butane and the second solvent is propane.

5. A process as defined in claim 1 in which the reservoir is maintained at a pressure in excess of 200 pounds/sq. in. and the gas is nitrogen.

References Cited in the file of this patent UNITED STATES PATENTS 2,623,596 Whorton et :al Dec. 30, 1952 2,669,306 'Ieter et a1. Feb. 16, 1954 2,669,307 Mulholland Feb. 16, 1954 2,718,262 Binder Sept. 20, 1955 FOREIGN PATENTS 849,534 Germany Sept. 15, 1952 ini 

1. A SECONDARY RECOVERY PROCESS FOR RECOVERING OIL FROM AN UNDERGROUND OIL RESERVOIR WHICH IS IN COMMUNICATION WITH AN INJECTION WELL AND A PRODUCTING WELL WHICH COMPRISES INJECTING A FIRST OIL SOLVENT DOWN THE INJECTION WELL AND INTO THE OIL RESERVOIR, SAID FIRST SOLVENT BEING A HYDROCARBON CONTAINING FROM 3 TO 5 CARBON ATOMS THEREAFTER INJECTING A SECOND OIL SOLVENT DOWN THE INJECTION WELL AND INTO THE RESERVOIR, SAID SECOND SOLVENT ALSO BEING A HYDROCARBON CONTAINING FROM 3 TO 5 CARBON ATOMS; THE COMBINED AMOUNTS OF THE TWO SOLVENTS BEING GREATER THAN 15 PERCENT BY VOLUME OF THE HYDROCARBON PORE VOLUME OF THE RESERVOIR AND THE SECOND SOLVENT BEING MORE VOLATILE THAN THE FIRST SOLVENT, MAINTAINING THE PRESSURE OF THE RESERVOIR AT A VALUE GREATER THAN THE VAPOR PRESSURES WHICH THE SOLVENTS NORMALLY POSSESS UNDER THE CONDITIONS THAT PREVAIL WITHIN THE RESERVOIR, THEREAFTER INJECTING A DRIVE GAS DOWN THE INJECTION WELL INTO THE OIL RESERVOIR, SAID GAS BEING NON-CONDENSIBLE UNDR THE CONDITIONS THAT PREVAIL WITHIN THE RESERVOIR, AND REMOVING FROM THE RESERVOIR THROUGH THE PRODUCING WELL OIL THAT IS SWEPT FROM THE RESERVOIR. 